A variety of respirator systems exist to protect users from exposure to dangerous chemicals. Examples of these systems include negative pressure or powered air respirators which use a cartridge containing a sorbent material for removing harmful substances from the ambient air, and supplied air respirators.
A number of protocols have been developed to evaluate the air being delivered to the user. These protocols may also be used to determine whether the sorbent material is near depletion. The protocols include sensory warning, administrative control, passive indicators, and active indicators.
Sensory warning depends on the user's response to warning properties. The warning properties include odor, taste, eye irritation, respiratory tract irritation, etc. However, these properties do not apply to all target species of interest and the response to a particular target species varies between individuals. For example, methylbromide, commonly found in the manufacturing of rubber products, is odorless and tasteless.
Administrative control relies on tracking the exposure of the respirator sorbent to contaminants, and estimating the depletion time for the sorbent material. Passive indicators typically include chemically coated paper strips which change color when the sorbent material is near depletion. Passive indicators require active monitoring by the user.
Active indicators include a sensor which monitors the level of contaminants and an indicator to provide an automatic warning to the user.
One type of active indicator is an exposure monitor, which is a relatively high cost device that may monitor concentrations of one or more gases, store and display peak concentration levels, function as a dosimeter through the calculation of time weighted averages, and detect when threshold limit values, such as short term exposure limits and ceiling limits, have been exceeded. However, the size and cost of these devices make them impractical for use as an end-of-life indicator for an air purifying respirator cartridge.
A second type of active indicator which has been disclosed includes a sensor either embedded in the sorbent material or in the air stream of the face mask connected to an audible or visual signaling device. The cartridge containing the sorbent material is replaced when the sensor detects the presence of a predetermined concentration of target species in the sorbent material or the face mask.
Some personal exposure indicators include threshold devices that actuate a visual or audible alarm when a certain threshold level or levels have been reached. In addition, some active indicators also provide a test function for indicating that the active indicator is in a state of readiness, e.g., the batteries of the indicator are properly functioning.
However, active indicators utilizing only one or two thresholds to activate alarms have constant characteristics after the alarm activation. These indicators provide no indication of the rate of change of target species above the threshold level, nor any sense of how long the user has to reach a safer environment or replace a respirator cartridge. Such constant characteristics are particularly disadvantageous because saturation of a respirator cartridge after attaining the threshold level can change rapidly due to a wide variety of factors, including temperature, humidity, and the nature of the target species. The lack of knowledge of the rate of concentration change could be a concern.
As shown in some devices, separate systems for indicating that the active indicator is in a state of readiness or that the active indicator is functioning correctly, have several disadvantages. In practical use, the user may forget, be unable to take the time, or not have hands available to press buttons or activate switches to verify the proper functioning of the indicator and/or the battery. Use of separate indicator systems for hazard alarm and readiness may also lead to a false sense of security, in that the separate hazard alarm could malfunction and the readiness alarm could still indicate that the active indicator is ready for use.
Additionally, if these systems use irreversible sensors, in which the property of the sensing device that indicates the presence of the target species is permanently changed upon exposure, once the sensing device is saturated, it must be replaced. Consequently, irreversible sensors if mounted in the sorbent material or the face mask must be shielded to prevent exposure to target species in the ambient air that are not drawn directly through the sorbent material. If the sensor is inadvertently exposed to the toxic environment, such as by a momentary interruption in the face seal of the respirator or during replacement, the sensor can become saturated and unusable.
For some applications, it is useful to identify decreasing concentrations of a target species, such as oxygen. Irreversible sensors typically are incapable of detecting decreasing concentrations of a target species.
Some disclosed indicators locate the sensor within the air flow path of the face mask so that it is not possible to detach the sensor or the signaling device without interrupting the flow of purified air to the face mask. In the event that the sensor and/or signaling device malfunction or becomes contaminated, the user would need to leave the area containing the target species in order to check the operation of the respirator.